Efficient Near-Infrared-Emitting Cationic Iridium Complexes as Dopants for OLEDs with Small Efficiency Roll-off

Two near-infrared- (NIR-) emitting cationic iridium(III) complexes, [Ir(pbq-g)(2)(Bphen)]+PF6- (1) and [Ir(mpbqx-g)(2)(Bphen)]+PF6- (2), were synthesized and characterized, where pbq-g, mpbqx-g, and Bphen represent phenylbenzo[g]quinoline, 2-methyl-3-phenylbenzo[g]quinoxaline, and 4,7-diphenyl-1,10-phenanthroline, respectively. By employing sp(2)-hybridized N opposite the chelating N atom in the cyclometalated ligand, we succeeded in expanding the emission of iridium complexes with simple structures to the truly NI R region of the spectrum. This subtle structural adjustment significantly lowered the LUMOs of the iridium complexes, leading to a 60-80-nm emission red shift of complex 2 relative to complex 1. Based on these solution-processable phosphors, NIR organic light-emitting devices (OLEDs) were fabricated with the emission covering the range 690-850 nm. Compared with common OLEDs, these NIR-emitting electrophosphorescent devices demonstrate exclusive small efficiency roll-off with increasing current density. For complex 1, the external quantum efficiency (EQE) was 0.67% at a current density of 6 mA/cm(2), and the value held at 0.61% at a current density of 20 mA/cm(2). In particular, the EQEs of devices based on complex 2 remained almost constant even up to 100 mA/cm(2). Such unique characteristics are desirable for the practical application of OLEDs in terms of energy savings. They can be ascribed to the bulky aromatic cyclometalated and ancillary ligands, together with the octahedral configuration of Ir(III) complexes, thereby hindering molecular aggregation and triplet-triplet annihilation under high populations of triplet excitons.